Players can choose from a small set of common races or roll for rarer race.
The way race selection works is fairly simple. Each race has a random assortment of distinct numbers from 1 to 10000. The more common the race, the more common it is in the list of 10,000. There's also a magic number in The List of 10,000 that causes a secondary roll for even rarer races. On the secondary roll the ultra-rare races are represented by five or fewer numbers, and at least half of the range is dedicated to re-roll from the first list.
It should also be noted that rolling for a rare race doesn't guarantee a rare race.